Much more accurate calls were made by Wells Fargo, Atlantic Equities , Macquarie Research, and Jack A. Bass ,who advised clients to get out of the high-flying stock about the time it peaked in December.

On Wednesday the stock fell as low as $37.24, 50 percent below its peak of $74.73 the day after Christmas, wiping almost $18 billion off Twitter’s market capitalization.

The downgrades, and the subsequent swoon by the stock, reflect concern about slowing growth in Twitter’s user base and the company’s ability to reverse the trend. Year-on-year growth in the number of Twitter users has fallen for five straight quarters, and the company said on Tuesday that its 255 million monthly users, on average, appeared to check the service less frequently than a year ago.

That in turn has fueled doubts that Twitter could one day attract as many users as Facebook Inc’s 1.2 billion, or match its much larger rival’s power as an advertising vehicle. It’s also raised questions over whether it can sustain growth over the long term. While no one is suggesting Twitter will lose its consumer cachet as happened to companies such as MySpace or Orkut, neither can anyone guarantee that as tastes change newer rivals won’t usurp it.

“Can they become a mainstream company? That’s the open question,” said Ben Schachter, the Macquarie Securities analyst who downgraded Twitter’s stock to “underperform” on December 27 – the day after it peaked.

It’s a far cry from the enthusiasm that greeted the company when it debuted on the New York Stock Exchange on November 7 and its shares soared 73 percent over the offering price. There was no let-up for the next two months, as the stock scaled fresh highs with little or no news to justify the valuation.

That got some analysts worried. Schachter, speaking to Reuters on Thursday, recalls a “runaway momentum.”